The present invention relates to an enhanced method for expanding and conferring a cup shape to the terminal junction segment of bi-axially oriented pipes made of thermoplastic material, by means of a calibration expander, able to expand the terminal segment, previously heated to the plastic state, shaping therein an annular circumferential seat for housing a corresponding sealing gasket for the junction.
As is well known, apparatuses that operate on thermoplastic materials such as PVC, polyethylene (PE) and/or polyolefins, which are not bi-axially oriented, comprise furnaces for the prior heating of the terminal segment. This pipe portion is brought to a temperature of about 120.degree. C. or more, to obtain its adequate softening and enables then easily to fit it on the calibration expander, which dilates it and shapes it into the so-called "cup" conformation, to obtain an effective junction between the pipe thus treated and the end of another pipe, at the nominal diameter, once between the two ends is interposed the annular sealing gasket, stably positioned in the annular seat obtained in said terminal segment. Within the scope of this technology the calibration expander, in this case said to be the mechanical expansion type, may present a crown of radially expandable sections able to be retracted on command, with the purpose of shaping the annular seat of the terminal segment when they are expanded, and to let the expander easily exit the shaped terminal segment, when they are retracted.
In this case it is evident that the annular gasket shall subsequently need to be inserted in the annular seat of the shaped and cooled terminal segment.
Alternately to the above, the calibration expander may present an annular groove able simultaneously to retain the annular gasket introduced therein with a thrust flange which, once the gasket is inserted into the expander, causes it to be housed in the annular groove. Once this is accomplished, the phase of shaping the terminal segment starts, with the necessary aid of a flange for upsetting and holding the gasket, located downstream thereof. Subsequently, the expander is extracted, and in this case it can be defined as the free gasket type, after cooling the terminal segment, whilst the gasket remains trapped in the annular seat formed in the terminal segment.
The aforesaid technologies refer, as stated, to the treatment of pipes made of thermoplastic material not bi-axially oriented.
The latter technology entails a considerable reduction in the thickness of pipes made of thermoplastic material, for the same resistance to internal pressure, since it has been ascertained that, in these materials, if molecules constituting the material are stretched or elongated in a same direction or in mutually orthogonal directions, the thickness of the material decreases, but not its resistance to the internal pressure designed for the untreated pipe.
Thus technologies have been developed for the bi-axial orientation of the thermoplastic material, entailing a circumferential dilation of the pipe and its longation or stretching in the axial direction. For example the international patent applications WO 95/25626, WO 95/25627, WO 95/25268, WO 95/30533 are mentioned. In this manner, for the same quantity or volume of material it is possible to produce a greater number of linear meters of pipe, which still meets the pressure resistance requirements originally prescribed.
However, such materials have an intrinsic characteristic which negatively reflects on the final product: if the extruded pipe initially has a certain diameter and, after the bi-axial orientation process, a clearly greater diameter which is the nominal one to be obtained, it tends drastically to reduce in diameter, if subjected to temperatures of a certain level, such as those able to soften the material for the processes whereby the terminal junction segment is formed according to the technologies illustrated above for pipes in non bi-axial thermoplastic material.
This is because the molecules of the thermoplastic materials that are subjected to diameter change "remember" the previous physical state and tend to return to the original state.
Hence, if a pipe is extruded to a certain diameter which is subsequently increased, in a bi-axial orientation treatment, if it were subjected to the temperatures (120.degree. C. and higher) of the aforesaid furnaces its diameter would drastically decrease.
The aforesaid technologies, perfectly suited to shape the terminal segments of pipes made of thermoplastic material not subjected to bi-axial orientation processes or anyway particular subsequent diameter dilations, are therefore not at all applicable to pipes made of bi-axially oriented thermoplastic material.
The bi-axially oriented pipe would lose its thinness characteristics, tending to return to the original thickness, and therefore could not be fitted in the expander set for the useful nominal diameter, corresponding to the diameter of the bi-axially oriented pipe.
Moreover, even if it were possible to reduce the heating temperatures of the thermoplastic material to limit diameter reduction, the end segment thus obtained still would not be able to fit on the expander, because it would be too cold and thus would exert an excessively high progressive friction.
The applicant has also observed that, since there is a need for the temperature within the differentiated temperature furnace to be limited, in order to prevent the inlet to the terminal segment to be reduced excessively and to retain insofar as possible unaltered the bi-axial orientation characteristics, there could be a risk of damaging the terminal segment during a particularly stressful use of the treated pipe. Moreover, the shaping phase on the expander, though appropriately heated, may become excessively onerous, for pipes of considerable size and technical characteristics of resistance, both in terms of effort required, and of operating times, thus penalizing the hourly production of the pieces.